Jacking Pipe

Jacking and Tunneling

In all jacking and tunneling operations, direction and distance are carefully established prior to beginning the operation. The first step is the excavation of jacking pits or shafts at each end of the proposed line.  The pit must be of sufficient size to provide ample working space for the jacking head, jacks, jacking frame, reaction blocks, spoil removal and one or two sections of pipe.  Provisions are made for the erection of guide rails in the bottom of the pit.  For large pipe it is desirable to set the rails in a concrete slab.  If drainage is to be discharged from the jacking pit, a collection sump and drainage pump are required. 
 
 The number and capacity of jacks depend on the type of soil, size of the pipe and jacking distance.  The jacks are placed on both sides of the pipe so the resultant jacking force is slightly below the spring-line.  Use of a lubricant, such as bentonite, to coat the outside of the pipe is helpful in reducing frictional resistance and preventing the pipe from setting when forward movement is interrupted. Because of the tendency of soil friction to increase with time, it is usually desirable to continue jacking operations without interruptions until completion.
 
Correct alignment of the guide frame, jacks and backstop is necessary for uniform distribution of the axial jacking force around the periphery of the pipe.  By assuring that the pipe ends are parallel, and the jacking force properly distributed through the jacking frame to the pipe and parallel with the axis of the pipe, localized stress concentrations are avoided.  A jacking head is often used to transfer the force from the jacks or jacking frame to the pipe.  In addition to protecting the end of the pipe, a jacking head helps keep the pipe on proper line by maintaining equal force around the circumference of the pipe.  Use of a cushion material, such as plywood, between adjacent pipe sections provides uniform load distribution.

Jacking Concrete Pipe

Concrete pipe is frequently installed by tunneling and jacking where deep installations are necessary or where conventional open excavation and backfill methods may not be feasible.  Concrete pipelines were first jacked in place by the Northern Pacific Railroad between 1896 and 1900.  In more recent years, this technique has been applied to sewer construction with intermediate shafts along the line of the sewer as jacking stations. The feasibility and planning of tunneling and jacking projects should be coordinated with local concrete pipe manufactures and jacking contractors.
 
Reinforced concrete pipe as small as 18-inch diameter and as large as 132-inch diameter has been installed by jacking.  Since conventional jacking procedures require access by workmen through the pipe to the heading, 36-inch diameter pip is generally the smallest practical size for most jacking operations.  When jacking smaller size pipe, earth removal is accomplished by mechanical means such as augers and boring equipment.
 
Concrete pipe is ideally suited for tunneling and jacking.  The pipe can be pushed forward immediately after the soil is excavated, providing a complete tunnel liner for protection of workers and equipment.  Because of technological advances and increasing experience, many pipelines are now being jacked.

Jacking Procedure

The usual construction sequence for tunneling and jacking concrete pipe is:

• Excavate jacking pits or shafts, construct jacking abutments or thrust blocks, and install jacks, jacking frame and guide rails.
• Begin tunnel excavation by machine, or hand, depending on conditions.
• Lower first section of pipe, position jacks and jacking frame, and jack pipe forward.
• Continue excavation, remove soil through pipe, insert seceding sections of pipe between the lead pipe and jacks and jack forward.
• Repeat sequence, excavation, soil removal, pipe insertion and jacking, until the operation is complete.

Excavation should not proceed the jacking operation more than necessary. Material is trimmed so the tunnel bore slightly exceeds the outside diameter of the pipe. Such a procedure results in minimal disturbance to the adjacent soils.  The lead pipe is generally contained within a shield projecting from the mining machine or equipped with a cutter or shoe to protect the pipe when excavating by hand.  Jacked pipe tends to set or freeze when forward movement is interrupted, resulting in significantly increased frictional resistance, therefore, continuous operation is desirable. Occasionally a lubricant, such as bentonite slurry, is pumped into the space between the tunnel bore and the outside of the pipe to reduce frictional resistance. After the jacked pipe have reached their final position, grout is frequently pumped into this same space to insure continuous bearing with the surrounding soil.
           
When increased resistance develops due to soil conditions or length of run, intermediate jacking stations may be inserted at periodic intervals. The intermediate jacking station pushes only the several lengths ahead while bearing on the pipe behind. The use of intermediate jacking stations reduces axial loads on the pipe and required jacking capacity.
           
The number and capacity of the jacks primarily depends upon the size and length of the pipe to be jacked and the type of soil encountered.  Abutments for the jacks must be strong enough and large enough to distribute the maximum capacity of the jacks to the soil behind the backstops.

Loads on Jacked Pipe

Two types of loads imposed upon concrete pipe installed by the jacking method are the axial load due to the jacking forces applied during installation and external earth and live loads.

Axial Loads

The axial or thrust loads are transmitted from one concrete pipe section to another through the joint surfaces. To prevent localized stress concentrations, it is necessary to provide relatively uniform distribution of the axial loads around the periphery of the pipe. This is accomplished by assuring that the pipe ends are parallel within the tolerances prescribed by ASTM standards, using a cushion material, plywood, between the pipe sections, and care on the part of the contractor to insure that the jacking force is properly distributed through the jacking frame and parallel with the axis of the pipe.

The cross-sectional area of a standard concrete pipe wall is more than adequate to resist stresses encountered in normal jacking operations.  For projects where extreme jacking pressures are anticipated due to long jacking distances or excessive unit frictional forces, intermediate jacking stations may be used, and greater care taken to avoid bearing stress concentrations.

Earth Loads

The major factors influencing the vertical earth load on pipe installed by jacking are:   

1. Weight of the prism of earth directly above the bore.
2. Upward shearing of frictional forces between the prism of earth directly above the bore and adjacent earth
3. Cohesion of the soil.

Live Loads
Jacked installations are generally constructed at such depths of cover that effects of live loads are negligible.  Highway and aircraft loads are considered insignificant at depths greater than 10 feet, however, railroad loads are considered up to 30 feet of cover.